The objective of this study is to learn more about the status of lysosomes in the process of amelogenesis by characterizing morphologically and biochemically the total pool of lysosomes contained within cells of the enamel organ and by investigating if changes occur in the volumetric size or in the levels of enzymatic activities of this pool in the course of amelogenesis. Three correlated experimental approaches are proposed. These include: (1) stereology to obtain data about the volumetric distribution of lysosomes among the various cell layers of the enamel organ at four time periods in amelogenesis corresponding to a proliferative stage, the start of enamel secretion, the finish of enamel secretion, and the period in the maturation stage where the loss of organic material from the developing enamel is maximal; (2) biochemistry to provide information about the gross activity of enzymes contained within the lysosomal pool of the enamel organ at the stage of enamel secretion as compared to the stage enamel maturation; and (3) electron microscopic cytochemistry to give a qualitative assessment of the ultrastructural sites of enzymatic distribution within the lysosomal pool at the stages of enamel secretion and enamel maturation. To facilitate analysis, the rat incisor is suggested as a useful experimental system for this investigation since the entire sequence of developmental steps in amelogenesis are present along the length of this tooth and the rat incisor has been defined precisely enough for reliable and reproducible sampling.